The invention relates to a semi-conductor device as set forth in the precharacterizing part of claim 1. Such a device is known from a publication by K. L. Anderson and L. M. Arzubi in the IBM technical disclosure bulletin Vol. 17, No. 6, November 1974 pages 1567, 1568.
This device uses a so-called 4T dynamic memory cell, that is, a dynamic memory cell which uses a cross-coupled pair of NMOS transistors for storing information and two NMOS access transistors for accessing the stored information. In the cross coupled pair, the gate of a first transistor is coupled to the drain of a second transistor and vice versa. The sources of the transistors are connected to each other.
This dynamic memory cell may be contrasted with a conventional 1T dynamic memory cell, that is, a memory cell with a storage capacitor and one access transistor for accessing information stored on the capacitor. A 4T memory cell allows a faster access and shorter cycle time than the 1T dynamic memory cell. On the other hand, a 4T memory cell requires a larger amount of substrate area for transistors than the 1T memory cell, but this is set-off by the fact that for use in a memory matrix the 1T memory cell needs a relatively large capacitor, which is not the case for a 4T memory cell. This makes it attractive to use 4T memory cells without such relatively large capacitors, especially in semi-conductor manufacturing processes that are not especially optimized for manufacturing memory capacitors, e.g. "logic" manufacturing processes designed for manufacturing signal processors, micro-controllers etc.
However, the use of a 4T memory cell without a relatively large memory capacitor has the disadvantage that leakage currents in the memory cell are more prominent, leading to a shorter retention time. The publication by Anderson et al. counters this problem by boosting the voltage that is stored in the memory cell. However, this puts special requirements on the voltage handling capability of the transistors, it complicates the circuit and provides only small gain.